


Capacious as the Sea

by tenrousei_kuroi



Category: Harry Potter - J. K. Rowling
Genre: (semi-explicit), Brother/Brother Incest, Brotherly Affection, Brotherly Love, Fluff, Fluff and Smut, If you're familiar with my work then you'll get why that's odd, Incest, M/M, Non-Canon Relationship, Not Canon Compliant, Protective Sirius Black, Sibling Incest, Sirius Black is a Good Sibling, Sirius Black is a sweetheart, Tender Sex
Language: English
Status: Completed
Published: 2020-01-20
Updated: 2020-01-20
Packaged: 2021-02-27 05:40:51
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,162
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/22332043
Author URL: https://archiveofourown.org/users/tenrousei_kuroi/pseuds/tenrousei_kuroi
Summary: Sirius catches a glimpse of something he didn’t even know he wanted. Sirius/Regulus
Relationships: Regulus Black/Sirius Black
Comments: 9
Kudos: 181





	Capacious as the Sea

**Author's Note:**

> Canon compliance: None whatsoever. This doesn’t fit in with any specific story universe of mine, but Sirius is like twenty and still living in Grimmauld Place so make of that what you will. 
> 
> Notes: Sirius/Regulus. Sirius is a sweetheart for once. That’s about all you need to know. Happy New Year! We’re getting this new decade started off right. ;)
> 
> Title notes: Capacious as the Sea  
> It’s such a depressing and moody title that doesn’t really match the tone of the story, but I’m oddly attached to it, so it’s staying.

_Reggie,_

_Mum and Dad left early for a weekend stay at the coast. Got the house to myself for once and I’ve got some fun lined up for this evening. Why don’t you come join me round about 7pm? I know you’ll be missing your evening astronomy class, but I promise it will be worth it._

_Love,_

_Sirius_

Sirius re-read his letter a few times before he sent it, making sure it wasn’t obvious what he was suggesting. You never knew whether or not one of those slippery Slytherins would sneak a look at Regulus’s mail, and the poor boy didn’t need it getting out that he enjoyed threesomes with his own brother.

Because that’s what the letter was about, make no mistake. Sirius had sent many in the past year and a half since all this...fun?... had begun. (Sirius tried not to think about how a year and a half ago Regulus had only been sixteen, which wasn’t something he was proud of. Although in the eldest Black’s defense, he had thought Regulus would take years of convincing, and had been rather taken aback by his brother’s relative bravery and eagerness.) So yes, a letter coaxing Regulus Black back home to Grimmauld Place at the behest of his brother was not uncommon.

The trick was all in how the letter was signed. ‘From Sirius’ or ‘Love Sirius’? Regulus would know the difference, he always did. Only this time...Sirius had something a little different in mind. Typically his letters weren’t lies. If he said he had a woman coming over who was down to have some fun with the both of them, he meant it. They’d done this many times. Regulus was getting more comfortable and more ambitious with it…

...but this time there really was no woman. Sirius’s hand was shaking a little as he returned his quill to his desk and re-read his letter one more time.

Because over the last few months, Sirius had come to realize that he paid his lady friends less mind with each midnight romp. It had gotten to the point where he flat out saw none of them unless Regulus was involved. And last week when he’d snuck his brother off to meet up with Marlene...he’d hardly even touched her. He’d just watched.

And he hadn’t been watching the slender brunete woman between them. No, his eyes had been glued to Regulus Black, the little brother who’d been so quickly growing up the past few years that Sirius almost couldn’t believe it.

It had taken awhile to come to terms with these strange feelings (and it had taken several full bottles of liquor to get to a point where he was willing to act on them) and now Sirius was ready to take some sort of dive, god damn it.

Sirius sent the letter off with his owl (tossing the poor, disheveled thing through the floo network to speed his message along), downed another drink and set about showering. Soon enough he heard the kitchen fireplace roar to life. A slow grin spread over his face. Regulus hadn’t missed a moment. Sirius dressed himself and set off down the hall.

“Welcome, welcome, little one,” Sirius sniggered, arms open and a bottle of vodka gripped loosely in one hand. “Thought you might be stopping by.”

Regulus sneered and rolled his eyes. He fell casually against Sirius in a sort of one-armed hug as he walked past him and towards the bedroom. Sirius laughed and spun around, following Regulus.

“You’re sloshed already,” Regulus observed. “Guess only one of us inherited any patience.”

“Here,” said Sirius as he pressed the bottle into Regulus’s hands. “Catch up with me.”

Regulus laughed. He took one drink but then passed the bottle back. “You know I prefer to remember my evenings.”

“Come on...be adventurous just this once? For me?” Sirius goaded his brother. Regulus rolled his eyes and threw himself onto Sirius’s bed.

“I think it’s pretty adventurous just to be here in the first place,” Regulus argued.

Sirius cocked his head to the side with a questioning look on his face.

“You know...sitting here, waiting to have sex with one of my brother’s girlfriends. While he’s in the room. Also having sex with them...don’t give me that look. This isn’t something most people would just jump right into, Sirius!”

Sirius gave Regulus a lopsided grin. “Doesn’t mean you can’t stand to loosen up just a little bit.”

“For god’s sake, Siri,” Regulus said as Sirius once again passed him the bottle of liquor. “I’m already here and willing, you don’t have to seduce _me._ ” He raised an eyebrow.

Sirius laughed in what he hoped was a casual manner. In reality, his heart was starting to race. He really wished Regulus would drink a little bit more. That would make him a bit more receptive to Sirius’s advances.

...advances?

“I’ll be right back. Gotta’ go make sure someone’s in the drawing room to greet our lady friend when she gets here.”

“I’ll come with?” Regulus offered.

“Nah, stay here and relax. Try to loosen up a bit, Reggie,” Sirius pointed again to the vodka.

Regulus rolled his eyes again but didn’t argue. Sirius left, his mind a whirling mess of nerves.

He wanted Regulus so badly...but he hadn’t exactly planned this thoroughly. What if Regulus didn’t want him back? Sirius could only hope this whole affair wouldn’t damage their relationship. Perhaps he could get Regulus drunk enough that he wouldn’t remember if things turned awkward…

...but no, plying his brother with alcohol wasn’t the answer. The thought of that kind of deception actually made Sirius feel a little sick inside...or perhaps that was just his stomach rejecting the massive quantity of vodka and firewhisky he’d already had tonight. For a moment, Sirius thought he might throw up—either from the alcohol or from nerves—and he high-tailed it to the bathroom.

Sirius leaned weakly over the bathroom counter, splashing some cold water onto his face. How foolish this entire venture had been! In his heart, Sirius felt a slow race of panic rising ever more steadily along with his heart rate. Was there any way to back out of this ridiculous, Firewhisky-fueled idea? What had his plan even been? To just waltz back into his bedroom, loudly announce that their third party member had been a lie and then lean in for a kiss?

If Regulus hadn’t realized something was amiss, he was surely set to soon. Sirius took a deep breath. This side of sober, the very concept seemed ludicrous. Had he really expected his brother to fall into his arms like this? Even the suggestion was likely set to ruin everything Sirius had with his brother.

Regulus had always put Sirius in a tricky position. As a child (and still now), he was hopelessly soft and gullible. Inside Regulus’s aloof, Slytherin shell was a gentle soul who’d dived in wholeheartedly when Sirius had pulled him by the hand into mutual debauchery. It had always seemed to Sirius that Regulus would follow him anywhere…

But there was a limit. There had to be a limit. If Regulus cottoned on that Sirius had ever thought about being with him...in this capacity and not just sharing a mutual lover, he would instantly become suspicious. Not just suspicious of Sirius’s current intentions, but of everything that had come before. Had Sirius perhaps set up this entire situation just to be with Regulus? Had he wanted not so much to bury himself in their shared lovers but to catch illicit glances of his own brother? To be as close to Regulus as was plausibly deniable?

...Perhaps he had. Sirius honestly couldn’t say when these feelings had started and whether or not this whole situation had begun with ill intent or if he’d honestly just wanted Regulus to loosen up a little.

And loosen up he had. Regulus’s confidence had improved with leaps and bounds over the last year. It hadn’t surprised Sirius when word had reached him that Regulus had been getting on quite well with the dating scene his final year at Hogwarts.

But it had enraged him, filled him with an unsettling emotion he realized now was jealousy.

Sirius shook his head vigorously and wiped his face off on a soft towel. There was nothing to be done for it, he needed to try and salvage this situation while there was still time.

He’d tell Regulus that his friend had canceled. Sirius was relieved that he had made up a fictional woman and not given her a name. It would be less likely that anyone would contradict him this way. Yes, he’d tell Regulus their date had canceled. Something had come up. Her brother was ill. She was ill. Her cousin needed her to babysit. She’d been called into work.

_Anything._

Taking several deep breaths, Sirius appraised himself in the mirror. He was a little flushed, but he could play that off as an effect of the alcohol.

Alcohol...he should offer Regulus a drink. He needed to come up with something for the two of them to do, Regulus had traveled all the way here, he’d skipped classes and everything.

Maybe Sirius could propose they break into Orion Black’s wine stores...drink and talk for awhile in their parents’ absence.

Sirius steeled himself and pushed open his bedroom door, praying...hoping...begging for Regulus to be sitting there with that gormlessly innocent expression on his face, the one that meant Sirius could fill his head with any nonsense and Regulus would believe him wholeheartedly.

“Hey, Reg...I...” Sirius’s voice stuttered to a halt.

Regulus was lounging on the bed, staring up intently at his brother, but his face was anything but innocent. There wasn’t a hint of confusion in Regulus Black’s expression.

Nor a stitch of clothing on his body.

Regulus had indeed done as Sirius had requested and gotten himself ready, only he didn’t seem even mildly concerned that Sirius had come back alone. In fact, a devilish grin was spreading across Regulus’s handsome face.

“Kept me waiting long enough, don’t you think?”

“Listen, Reg,” Sirius stammered. “She—she, er...you see, something came up and...” Sirius voice trailed off desperately as Regulus stood, satin sheets falling free of his body, and walked smoothly to the doorway. He stopped just shy of Sirius and reached one hand up to cautiously brush his warm fingertips across his brother’s lightly stubbled chin.

“Couldn’t even shave for me, could you?” Regulus chided playfully.

“Regulus, I was just going to tell you that...”

Regulus arched one perfect eyebrow. “Tell me what? Some amusing tale about how your lady friend was summoned away at the last minute?”

Sirius nodded. He was finding it difficult to breathe at the moment. “Yes, so, actually...”

Regulus put on a theatrical pout. “You’re not going to send me away, big brother, surely? After all, you’ve booked me for the whole evening and I’m already here...”

Sirius felt his mouth go dry. “You don’t...I...you can’t mean..?”

Regulus pressed his soft body against Sirius’s chest. Sirius felt himself break out in a hot sweat.

“I’ve been waiting for this day,” Regulus admitted coyly. He wrapped one of Sirius’s hands in his own. “Each time you’d call me, I hoped...and then finally...”

Sirius shook his head in disbelief. Regulus leaned forward to gently kiss his shoulder before sauntering back to the bed, where he lounged against the headboard, a delicious sight, indeed. Sirius felt his eyes move of their own accord, tracing Regulus’s smooth muscles, his slim waist, the light pattern of hair that trailed down his belly…

“S—Sirius?” Regulus folded his legs up and leaned forward, a flicker of uncertainty crossing his face. Sirius realized he’d been standing gobsmacked in the doorway for quite some time. “I’ve not...made a mistake, have I?”

Still Sirius was silent, staring. Regulus fidgeted. “Perhaps I should go...”

But his words were cut short. Sirius finally snapped out of his reverie, bounding forward and grabbing Regulus by the shoulders. Leaning down, he kissed his brother full on the lips, pressing, searching…

Regulus let out an undignified squeak and toppled backwards onto the bed, Sirius clambering after him, shucking clothes as he went. He wanted to touch, to kiss, to feel as much as he could before he awoke from whatever dream this surely was.

Regulus gasped for breath in between the assaults from Sirius’s lips. With a wild chuckle, he spat out Sirius’s hair as it entangled his face and he reached blindly for Sirius’s trousers, helping him slide them off. Soon Sirius was naked atop him, grinding down deliciously. Regulus keened as their skin made contact. He’d been _near_ Sirius many times, but never _with_ him. He’d never had the whole of Sirius’s affections to himself, and it felt glorious.

“What gave me away?” Sirius asked as he made his way down Regulus’s body, planting hot kisses every few inches as he went.

Regulus shrieked when Sirius nipped at the ticklish skin near his hip. He rolled onto his side, panting.

“You told me,” he explained breathlessly. “In your sleep, I heard you say it.”

Sirius grinned. That his prolific sleep talking had finally been the harbinger of something _good_ was nothing short of amazing.

“So you knew this whole time just what I intended,” Sirius accused playfully. “And still you let me sweat in my own panic all evening, you prat.”

Regulus rolled back over underneath his brother and leaned over to shyly kiss Sirius’s forearm. “Well there was always the chance that I had misunderstood. I was pretty sure, but still...”

Sirius bit lightly at the shell of Regulus’s ear, breathing deeply the fresh scent of Regulus’s hair conditioner and the clean smell of his skin. “Well I hope you know what you’ve gotten yourself into, Reggie, because I intend to make good on my sleep promises,” said Sirius smugly.

“Oh, yeah?” Regulus asked.

Sirius kissed him again. His whole body was giddy, tingling with anticipation and sheer joy. “Absolutely,” he said, reaching a featherlight hand to Regulus’s waist, gently trailing fingers along his skin until he just barely grazed the base of Regulus’s stiff cock. Regulus mewled in excitement.

Sirius leaned back for a moment. He was a tad nervous. He’d never been with a man before, and he had every intention of giving Regulus the ride of his life. But Regulus seemed nothing short of ecstatic, wriggling under Sirius and demanding more kisses.

Sirius pulled his bottle of lubricant from the nightstand and coated up his hand before setting to work on Regulus, gently rubbing and palming his cock until Regulus was hot and panting.

“Do you like that?” Sirius cooed, swiping a soft hand across Regulus’s chest.

Regulus nodded.

“Want me to keep going?” Sirius asked with a sly smile. Regulus raised an eyebrow.

“Already decided how things are going to stack up, have you?”

Sirius’s eyes widened and he scoffed with amusement. To hear Regulus so brazenly questioning him was a rare sight, indeed!

“Oh, did you have other plans?”

“Yeah,” said Regulus indignantly. “Maybe I wanted to fuck you!”

Sirius actually laughed out loud and Regulus smacked his arm petulantly.

“Typical,” he said while Sirius sniggered into his shoulder. “You assume just because you’re older than me...”

Sirius finally reeled in his laughter and sat up, rubbing calming circles on Regulus’s inner thigh. “Sorry, Reggie,” he said earnestly. “Tell me what you want.”

Regulus threw himself backwards into the pillows with a huff. “Fine, fine,” he conceded. “I won’t be responsible for you pouting all evening. I guess I could be persuaded to go along with your plans...but you’ll owe me.”

It was Sirius’s turn to cock an eyebrow. “Oh, will I?” he asked. “And just what might that be?”

“I’ll tell you when I decide to collect,” Regulus said. Then he crawled back to Sirius and started to kiss him again. “Well go on then, big brother,” he continued with theatrical enthusiasm. “Take me, I’m yours...or something.”

Sirius set about preparing things quickly. He didn’t want to lose out on the heat of the moment, but he was also very nervous about hurting his brother. If Regulus had a bad time…

“Stop worrying, Siri,” Regulus said, arching his back as Sirius’s probing fingers stretched him out. “I know you won’t hurt me. And I’d never be scared of you.”

Sirius shook his head in disbelief. “They been teaching legilimency down there in the Slytherin dungeons, or what?”

Regulus sniggered.

The clock in the hall started to chime 10 p.m. Regulus leaned over and blew out Sirius’s bedside candle, leaving them to bathe in the glow of the fire and the light of the moon. Sirius found himself temporarily lost in the sheen of Regulus’s skin as the moon’s shine and the fire’s embers seemed to dance across it. Shadows in the conclaves of Regulus’s muscles and the dips in his perfect form were enticing, like mysterious hollows, and Sirius kissed each one he found.

Regulus was so much better than anyone Sirius had had before. Despite his nerves and inexperience, Sirius had never felt so good. Regulus helped contort himself to an appropriate angle and then slowly, Sirius fell into him.

“Fuck...” he breathed into Regulus’s neck.

Regulus keened and prompted Sirius to move. Sirius wanted to remind Regulus to tell him if anything was wrong, if he was hurting him at all...but he was too lost in the most intense pleasure he’d ever felt. A part of Sirius’s brain felt a small sadness. He was used to taking care of Regulus, protecting him, but Regulus had made it quite clear that he had things under control.

Was it possible that Regulus no longer needed his older brother? A small grief, a feeling of loss, curled around in Sirius’s belly at this changing dynamic. But there was happiness, too, and a burgeoning feeling of excitement. Sirius rolled his hips languidly into his brother, stroking him as he went. Regulus moaned and looked up at him, clearly both grateful and surprised by Sirius’s tenderness.

Sirius cradled Regulus’s head in his arms and gently kissed his neck, whispering, “Why so surprised? You didn’t think I was just going to fuck you and leave you hanging?”

Regulus giggled and then arched his back, his breath hitching as Sirius jabbed at something very pleasurable inside him. “Yeah, I thought you’d be rougher. You’re usually so...I don’t know...harsh with everyone. Never seen you be sweet before.”

Sirius leaned up again and set himself to work trying to coax an orgasm from his brother. He really wanted Regulus to come first, but he needed it to happen quickly. He tried to recall every memory he had of the things he’d seen Regulus enjoy the most from their other partners. “Maybe you’re just the first person who’s mattered.”

Regulus gasped and for a moment, Sirius worried he’d hurt him, but then he felt Regulus tighten around him, saw his eyes flutter and felt his hips jerk between them. Regulus covered his mouth as he came, trying in vain to stifle his moans.

Sirius held Regulus by his hips and rode out his brother’s release. Only when Regulus stilled, sweat glistening on his neck and a hazy look in his eyes, did Sirius allow himself to focus again on his own pleasure. Although now he was keenly aware that it was his turn to be fast. Regulus was coming down from his high, and Sirius didn’t want to cause him any discomfort.

“Fuck,” Sirius muttered. He could feel his own hair now clinging to his face and neck. No one else had ever been this good. He wanted to go faster, harder...but he held himself back. This was Regulus; he couldn’t risk hurting him.

“Mmm, I love this,” Regulus whined, raking a hand down Sirius’s chest. Sirius felt sparks fly from his brother’s fingers.

“What? Me being gentle?” Sirius found himself probing, needing to know exactly what Regulus wanted from him.

Regulus nodded. “S’nice. At least for tonight...and I supposed we have all the time in the world for you to bend me over the desk and drill me hard and fast like you so clearly want to...”

Regulus’s words did the trick. Before he could even think of prolonging their lovemaking, Sirius felt himself slip right over the edge. There were stars flickering around his vision. Everything was somehow bright in the darkness. Regulus’s voice faded out for a moment and all Sirius could hear was a pleasant humming as he braced himself above his brother, trying with all his might not to collapse onto him.

As his high began to settle, Sirius slowly pulled out...gently, wary of any pain his movements might cause his brother. Suddenly he was a bit worried...he’d not had the presence of mind to ask Regulus if he had wanted him to pull out...

“Sirius,” Regulus whispered and Sirius was pulled from his worried reverie. He looked down to see Regulus was staring up at him in pure adoration. Sirius stilled for a moment and brushed Regulus’s sweaty fringe out of his eyes.

“I love you,” Regulus admitted. “And...I don’t want to be with anyone else.”

Sirius’s heart nearly stopped. Things were progressing so quickly, and Regulus...could Regulus really be already answering the questions Sirius hadn’t thought he had the bravery to even ask? Sirius’s plans hadn’t really gone further than getting Regulus into bed. It had seemed to his alcohol-addled mind that all the pieces would immediately fall into place after that.

...could it be that they were?

Briefly, Sirius wondered if he was imagining his brother’s words. He had drunk a lot...but no, Sirius had been coming out of his drunkeness since Regulus had arrived. This was real. This was happening.

“Do you...do you still want to see other people?” Regulus asked shyly, and with a start, Sirius realized that his long pause had clearly begun to worry his brother.

Sirius actually laughed out loud. The sound startled Regulus, who for a moment looked once more like the scared and nervous child Sirius remembered him being. Sirius composed himself and gave Regulus a warm smile to calm him down.

“You even have to ask?” he said. “Regulus, if I thought I could get away with it, I’d never leave this room again.”

“Really?” Regulus gasped. He sat up against the headboard, and flicked on the lights with a talented wave of wandless magic, not in the slightest ashamed of his nudity. Sirius glanced quickly at the sheets around them and at Regulus’s legs, frantically searching for blood or any other sign he’d been too rough with his brother. His heart calmed as his eyes found nothing. “I mean, it’ll be hard,” Regulus continued nonchalantly. “We’ll have to keep it a secret.”

Sirius laughed again, his voice more like a bark than ever.

“I’ve always been good at being sneaky.”

Regulus rolled his eyes at Sirius’s self aggrandizing, boisterous tone. A loud swish and the sound of muffled voices drifted from downstairs. “Well, you’d better start practicing right now, big brother, because that sounds like Mum and Dad coming back home early.”

Regulus lounged against his brother’s pillows, one arm propped behind his head, and watched Sirius Black dash furiously about, throwing on clothes and trying vainly to smooth his hair.

“Oh, yeah, just sit there and gloat, you little smart ass,” Sirius snapped, but there was a goodnatured tone to his voice. “Mum and Dad will kill me if they find you here, skipping class.”

“Then you better make sure they don’t realize I’m here,” Regulus said with a smirk. Sirius glared at him. A moment later, Orion Black’s voice was calling for Sirius...from a disturbingly close distance.

Regulus chuckled as Sirius, with his shirt on backwards and his face hopelessly disheveled, leaped out into the hall, closing the bedroom door behind his back and trying to head their father off.

Regulus lifted a hand over his mouth to stifle his own laughter. “Have fun, Siri,” he sniggered.

After a moment, Regulus climbed leisurely out of bed and got dressed. With an almost unbearably happy heart, he set about penning a letter at Sirius’s desk, breaking things off with the several casual relationships he had cultivated at Hogwarts in recent months. They were all nice, easygoing young women, and Regulus knew they’d be happy for him finding a...a…

...boyfriend?

Husband?

Special someone?

Regulus almost laughed again as each noun he wrote seemed equally as silly as the last. He cocked an ear and heard Sirius and their parents discussing something financial in the study down the hall, Sirius’s voice sounding particularly high-pitched. Yes, Sirius Black was a special someone, all right.

And he was all Regulus’s.

**Author's Note:**

> Look at how happy and fluffy this was. Something must be wrong with me…For once I can actually say that I do condone the portrayed relationship (I mean, not the incest part, obviously, but at least the dynamic. Look, I’m trying, okay? >.<). 
> 
> I’ve finally got some free time again, at least until grad school starts up in the summer. So if you have a story request, now’s the time ;)


End file.
